A nanoparticle based sensor in which smaller particles are seeded at a higher density to produce a faster response time than that of a sensor using larger particles and less dense seeding. The nanoparticles may comprise palladium nanoparticles. The sensor may be used in hydrogen fuel cells.
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This application is a continuation-in-part of U.S. patent application Ser. No. 11/551,630, which claims priority to U.S. Provisional Application Ser. No. 60/728,980 and PCT Application PCT/US2006/030314, all of which are hereby incorporated by reference herein. This application is a continuation-in-part of U.S. patent application Ser. No. 10/854,420, which claims priority to the U.S. Provisional Application Ser. No. 60/475,558 and PCT Application PCT/US2006/030314, which claims priority to the following provisional patent applications: Ser. Nos. 60/728,353 and 60/728,980, all of which are hereby incorporated by reference herein. This application also claims priority to U.S. Provisional Application Ser. No. 60/793,377, which is hereby incorporated by reference herein.BACKGROUND
Sensors using palladium metal for gaseous hydrogen sensing is a two step process, wherein the diatomic hydrogen molecule dissociates into monoatomic hydrogen in the surface of the palladium metal and the monoatomic hydrogen diffuses into the palladium lattice causing a lattice expansion in palladium (up to 5%), triggering a phase change (see
FIGS. 13(a)-(b) illustrate a change of resistance of hydrogen sensors;
FIGS. 14(a)-(b) illustrate initial resistances of sensors; and
A problem to be solved is to find a range of particle size and density for a fast hydrogen gas sensor. Disclosed herein is a range of particle size and density that achieves a response time of 10 seconds or lesser at high hydrogen concentrations.
Another approach has been disclosed in U.S. Pat. No. 6,849,911, which is incorporated by reference herein, for the creation of a palladium based hydrogen sensor, by fabricating a network of palladium nanoparticles on a resistive substrate by an electrochemical deposition technique. As the palladium nanoparticles expand on a resistive substrate between two electrical contacts, they short out miniscule resistances in the resistive substrate which happen to lie beneath two adjacent nanoparticles. On a large-scale statistical basis, the end-to-end resistance of the substrate then decreases in proportion to the amount of hydrogen. This sensor therefore measures hydrogen, rather than just detecting its presence.
(a) Palladium Nanoparticles Networks Versus Thin Films or Nanowires (Prior Art)
A thin film of palladium is a continuous surface, with normal metallic connection between atoms. The response of thin-film palladium to increasing levels of hydrogen has a positive coefficient. That is, the resistance increases with increasing hydrogen concentrations (see
(b) Use of a Resistive Substrate and Palladium ‘Nano-Switches’ (Prior Art: U.S. Pat. No. 6,849,911)
Uses nanoparticles on a resistive substrate as known in prior art (see
(c) Characteristics of a Proper Resistive Layer (Prior Art: U.S. Pat. No. 6,849,911)
Certain requirements are imposed on the resistive layer on which the nanoparticles are formed. It should ideally be stable with temperature, should be insensitive to environmental factors, should accept the formation of the nanoparticles. It further yields a certain ‘non-exposed’ resistance that is optimal for the electronics to which it connects. For the case of the sensors and electronics, the optimum resistance of a 0.5 mm×2.0 mm resistive surface yields a resistance range of 1200 to 2200 Ohms.
The optimum value is determined by desired operating current, impedance-based immunity to nearby electrical signals, and by resistive stability of the surface. If a surface such as titanium is used, thicker surface films improve aging characteristics but diminish both resistance and available signal. If that same film is too thin, electrical noise increases and the film is less immune to effects such as oxidation, for which titanium is otherwise notorious. The optimal resistance for the above physical configuration is 90 to 150 angstroms of titanium. The actual choice of resistive film material does not alter the means and methods of this patent. Each material brings with it physical characteristics that can be compensated for using the general means of this patent.
(d) Nanoparticle Fabrication on a Resistive Substrate (U.S. patent application Ser. No. 10/854,420 which is incorporated by reference herein)
The palladium nanoparticles are fabricated on a resistive substrate by an electroplating method. The electroplating bath comprises 0.1 mM PdCl2 and 0.1 M HCl dissolved in water. The process of electroplating the nanoparticles is necessary for successful operation of the sensor that nanoparticles have a certain distance between each other within a narrow distance window.
If inter-particle spacing is large, the sensor will be both slow and insensitive to low concentrations. Indeed, there will be a minimum threshold, for both temperature and concentration below which the sensor will not function. This is because the particles are spaced too far apart to touch each other, even at their times of greatest expansion and growth.
It is therefore necessary to control both the nano-particle size and the seeding density on the substrate. In this invention, palladium nanoparticles are grown by a two step plating process involving a short nucleation pulse (generally <10 sec) and a longer growth pulse (<10 minutes). The nucleation and growth parameters are controlled in the electrochemical fabrication process to produce functional sensors in different hydrogen concentration ranges. The density of the nanoparticles are generally controlled by the charge in the nucleation step (short pulse) and the size of the particles are controlled by the growth step (long pulse). A typical plating curve is shown in
The speed of the sensor (referred to as response time) can be controlled by controlling the size of the nanoparticles.
Thus a problem to be solved is to find a range of particle size and density for a fast sensor. Disclosed herein is a range of particle size and density that achieves a response time of 10 seconds or lesser at high hydrogen concentrations.
Identification of a Range of Nanoparticle Size and Density for a Fast Responsive Hydrogen Sensor
It can be seen that the (100-SL) sensors have a particle size of around 50 nm and an interparticle distance of around 150 nm. The SEM micrographs are shown in
It can be seen that the (100-SN) sensors have a particle size of around 50 nm and an interparticle distance of around 30 nm. The SEM micrographs are shown in
It can be seen that the (100-SH) sensors have a particle size of around 20 nm and an interparticle distance of around 1-2 nm. The response time (t90) of the sensor was around 25 seconds for 400 ppm H2. The SEM micrographs are shown in
It can be seen that the (100-NN) sensors have a particle size of around 50 nm and an interparticle distance of around 30 nm. The response time (t90) of the sensor was around 35 seconds for 40000 ppm (4%) H2. The SEM micrographs are shown in
The ratio of particle diameter (d) to interparticle distance (l) of the 100-SH type is around 0.85 to 1.0 and that for the 100-NN type is around 0.6 to 0.85. Thus the particle diameter (d) to interparticle distance (l) of the nanoparticles determines the speed of sensor.
Thus, the particle size and densities were varied for pure Pd sensors to achieve a faster response time. Concluded is that a sensor with higher particle density and smaller size (100-SH) improves the sensor performance in terms of response time.
The hydrogen sensor may be made by a glass substrate cleaned and metal film deposited on it. After that, it is patterned and contact pads deposited. The detecting part of sensor is made through wafer dicing, electroplating and chip dicing. The whole unit of sensor may be about 1 cm×1 cm and detection part smaller than 0.5 cm×0.5 cm. The palladium or palladium-silver composite particles are supported on base. The particle size may be about 100 nm. The particle size and particle packing density may be varied as shown in Table 1. The composition of metal was 100% of palladium or the ratio of palladium and silver being 90:10. These particles were arranged as several belts of each width being 10 μm.
The performance of the hydrogen sensor was tested.
The relative order for 90-SN and 100-SN was not revealed. However, on the whole, the addition of silver would inhibit the embrittlement by hydrogen and the responsibility of sensor decreased. Next, the effect of particle size. In the case of 4% hydrogen, the responsibility was almost constant regardless of particle size (between 100-NN and 100-SN, 90-NN and 90-SN). In the case of 400 ppm hydrogen, the responsibility increased with the increasing particle size. In the particle size of this study, the large particle size seems to be desirable for high responsibility.
Above, the 100-SN type sensor shows the highest responsibility in any case. Next evaluated are the effect of temperature and hydrogen concentration of 100-SN type sensor in detail.
In particular, the responsibility of 80° C. was significantly higher than that of 60° C. At 80° C., the relative difference of resistance was about 0.9 within 10 seconds. This high responsibility was because the increase of temperature probably made the diffusion rate of hydrogen atom in palladium composite metal higher and leaded to fast swelling of metal to give high responsibility of sensors.
Developed are several type hydrogen sensors by using palladium nano-particles, evaluated over a wide temperature range and hydrogen concentration. The sensor detected hydrogen by the change of resistance related to the swelling of palladium and the resistance of sensor decreased under hydrogen atmosphere. This hydrogen sensor detected hydrogen concentration over a range from 400 ppm to 4% regardless of the particle size and particle packing density. On the whole, the responsibility of the sensor made from 100% palladium was higher than that made from 90% palladium-10% silver composite. Further, the increase in particle packing density promoted the response of sensor. The increase in both temperature and hydrogen concentration significantly increased the responsibility of sensor, which is probably because the diffusion rate of hydrogen in palladium increases with temperature and the difference of partial pressure between inside and outside of particles.
The substrate material may be titanium, although this may be replaced with less-reactive vanadium. One skilled in the art will appreciate that various other materials could be used, including organic materials, so long as they fit the resistivity and operational ranges, and material compatibility issues for the sensor as a whole.
Titanium is a quite reactive metal, and must be well understood to be useful in a sensor application such as this. Referring to
To minimize oxidation-based aging in the field, the sensors may be pre-oxidized by subjecting them to an elevated temperature in an oxygen atmosphere. For example, the resistive Ti film may be 100 Angstroms thick when created. Oxidation may reduce that thickness to perhaps 80 Angstroms, for example, replacing 20 Angstroms by TiO2, an insulator.
While the oxidation continues indefinitely, it gradually becomes a much slower process as the oxide thickens, because large O2 molecules are required to penetrate far deeper than at the start of the process.
To control the aging, the Ti layer may therefore be thickened so that it can be corrected back by the thinning process of pre-oxidizing it. Therefore, thicker films of 150 Angstroms, for example, may be used instead of thinner 90 Angstroms, for example. The trade-off is that it provides a lower initial resistance.
The reference element (
For the active element, two palladium mask types may be used, solid-fill (
1. A sensor comprising nanoparticles that expand in a presence of hydrogen, the nanoparticles having a particle size less than 50 nanometers, the nanoparticles deposited on a substrate with a density where an average ratio of particle size to a distance between particle centers is in a range of 0.85 to 1.00.
2. The sensor as recited in claim 1, wherein the nanoparticles comprise palladium nanoparticles.
3. The sensor as recited in claim 1, wherein the nanoparticles are palladium nanoparticles.
4. The sensor as recited in claim 1, wherein the nanoparticles comprise palladium and silver.
5. The sensor as recited in claim 2, further comprising two electrodes positioned at ends of the substrate for sensing a current running through the substrate and nanoparticles.
6. The sensor as recited in claim 5, wherein the substrate is resistive.
7. The sensor as recited in claim 1, wherein the nanoparticles have a particle size in a range of 20-30 nanometers.
8. The sensor as recited in claim 1, wherein the sensor is operated in a temperature range of 0 to 100C.
9. The sensor as recited in claim 1, wherein the sensor is operated in a temperature range of 60C. to 90C.
International Classification: G01N 33/00 (20060101); G01N 27/00 (20060101);